1. Field of the Invention
The present invention relates to a catalytic hydrodealkylation method for producing .beta.-methylnaphthalene, and more particularly to a method for producing .beta.-methylnaphthalene of a chemical grade with a high yield from a feed oil containing an alkyl naphthalene having at least two methyl groups.
2. Related Art
.beta.-Methylnaphthalene is a compound having a boiling point of 241.1.degree. C. and a melting point of 34.4.degree. C. At room temperature, it is a colorless solid in the form of crystals. It has conventionally been obtained, by means of being separated through crystallization operation from fractions having a boiling point around 240.degree. C. of coal tar. Alternatively, it has been produced through vapor phase methylation reaction of naphthalene with methyl chloride, or through reduction of 2-methyl-1,4-naphthoquinone.
Recently, .beta.-methylnaphthalene has become more noticeable and valuable as a starting material for producing polyesters in commercial base. However, conventional methods for producing .beta.-methylnaphthalene, including separation from coal tar cannot supply it with sufficient amounts to comply with its demand. Moreover, .beta.-methylnaphthalene prepared by such conventional methods is not satisfactory in terms of purity for industrial use.
In the other words, it is not technically and economically practicable to implement mass-production of .beta.-methylnaphthalene of a chemical grade, i.e., .beta.-methylnaphthalene having a purity of not less than 97-98% and containing reduced amounts of such non-hydrocarbon impurities as nitrogen, and sulfur by means of conventional experimental approaches for producing .beta.-methylnaphthalene.
Accordingly, a new process applicable to industrial production of .beta.-methylnaphthalene is necessary to comply with the requirements of supplying it at low costs.
It is also noted that, since .beta.-methylnaphthalene was previously not demanded in great amounts as an industrial starting material, attempts to produce .beta.-methylnaphthalene industrially have not attracted much attention to date. As a result, no attempts other than those experimental approaches mentioned above have been made for developing a new method for producing .beta.-methylnaphthalene, especially that of chemical grade.
The present inventors searched an inexpensive feed oil which is readily available in petroleum and petrochemical industries and is replaceable for coal tar which has conventionally been used, and noticed an intermediate product obtained through various kinds of refining processes of a crude oil, particularly an intermediate fraction having a high content of naphthalene compounds, which is generated in catalytic-cracking heavy fractions of a crude oil to produce a gasoline stock.
The present inventors attempted to separate and collect .beta.-methylnaphthalene directly from the intermediate fraction, and concluded that it has been technically difficult to obtain .beta.-methylnaphthalene of a high grade having high purity and containing reduced amounts of non-hydrocarbon compounds such as sulfur-containing compounds and nitrogen-containing compounds, i.e., so-called .beta.-methylnaphthalene of a chemical grade. This is firstly because such an intermediate fraction contains too small a ratio of .beta.-methylnaphthalene against the whole naphthalene compounds to obtain it industrially, and secondly because, as shown in a gas chromatography chart of FIG. 2A, an intermediate fraction contains multi-fractional hydrocarbon components, including not only .beta.-methylnaphthalene but also significant amounts of a variety of hydrocarbons having boiling points close to the boiling point of .beta.-methylnaphthalene, as well as non-hydrocarbon impurities such as sulfur-containing compounds, nitrogen-containing compounds, etc.
Therefore, separation of .beta.-methylnaphthalene from the intermediate fraction is technically very difficult and is not economically practicable.
In the course of research, the present inventors found out that .beta.-methylnaphthalene was able to be effectively mass-produced with a significant reduction in the amount of impurities contained in .beta.-methylnaphthalene product by catalytic hydrodealkylating of a feed oil containing alkyl naphthalenes such as the above-mentioned intermediate fraction. The present inventors conducted further research to conclude that this approach enables the conversion of a feed oil into an product oil containing an elevated content of .beta.-methylnaphthalene and having such distillation properties that allow easy separation of .beta.-methylnaphthalene of a high grade and that the approach is economically quite advantageous and attractive.
The present inventors examined known hydrodealkylation processes for alkyl aromatic compounds such as those described in Japanese Patent Application Laid-Open (kokai) Nos. 206686/1990, 298347/1990, and 304033/1990, to evaluate those applicabilities to .beta.-methylnaphthalene production and found that these conventional hydrodealkylation processes are not applicable to the production of methylnaphthalenes having a specific methyl group, particularly .beta.-methylnaphthalene, although these methods are capable of producing naphthalenes by complete dealkylation.
By the way, the invention disclosed in Japanese Patent Application Laid-Open No. 206686/1990, "Hydrodealkylation Process" has been filed to U.S. Patent Office and allowed as U.S. Pat. No. 5,132,480 dated Jul. 21, 1992, providing a process for catalytic hydrodealkylation of a feed oil having alkylaromatic hydrocarbons, wherein a two-column apparatus, one column for the catalytic hydrodealkylation and the other column for regeneration of the catalyst, is used, which comprises conducting the hydrodealkylation of a feed oil in a first column under the presence of a fluidized bed of a catalyst comprising substantially spherical particles having a weight mean diameter of 25 to 250 .mu.m, an apparent density of 0.3 to 1.5 g/cm.sup.3, a pore volume of 0.10 to 1.5 cm.sup.3 /g under the conditions maintained for the hydrodealkylation in the column of a total pressure of 2 to 30 kg/cm.sup.2, a hydrogen partial pressure of 1.5 to 20 kg/cm.sup.2 and a temperature of 350.degree. to 700.degree. C.
The invention disclosed in Japanese Patent Application Laid-Open No. 298347/1990, "Hydrodealkylation Process" has been filed to U.S. Patent Office and allowed as U.S. Pat. No. 5,053,574 dated Oct. 1, 1991, providing a process for catalytic hydrodealkylation of alkylaromatic hydrocarbons which comprises contacting an alkylaromatic compound under a hydrogen partial pressure of 1 to 50 kg/cm.sup.2 and at a temperature of 450.degree. to 700.degree. C. with a catalyst which comprises porous alumina particles with coke deposited thereon in the pores, said alumina particles having a pore volume of 0.1 to 1.5 cm.sup.3 /g and a specific surface area of 5 to 500 m.sup.2 /g, the quantity of said coke being 1 to 30% by weight of said alumina particles, and the pore volume and the specific surface area of said catalyst being 0.05 to 1.5 cm.sup.3 /g and 1 to 500 m.sup.2 /g, respectively.
Japanese Patent Application No. 304033/1990 has disclosed a process for catalytic hydrodealkylation of alkylaromatic hydrocarbons which comprises contacting an alkylaromatic compound at a temperature of 450.degree. to 680.degree. C. under the presence of hydrogen with a catalyst which comprises vanadium (V) carried by alumina particles.